Solid immersion lens structures and methods for producing solid immersion lens structures

ABSTRACT

A solid immersion lens structure is a translucent pliant elastomer cast to a desired shape and smoothness. A method for construction of a solid immersion lens structure includes providing a mold defining a lens shaped cavity in which a solid immersion lens is cast, casting a translucent liquid elastomeric material into the lens cavity, permitting the elastomeric material to set to form the solid immersion lens portion and removing the solid immersion lens portion from the mold. A specific material for use as the solid immersion lens is a translucent silicone elastomer of a refractive index greater than n=1.4, such as General Electric RTV 615.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

The U.S. Government has certain rights in this invention pursuant toGrant No. HG 01642 awarded by the National Institute of Health and underContract No. PHY-9722417 of the National Science Foundation.

TECHNICAL FIELD BACKGROUND OF THE INVENTION

The present invention relates to solid immersion lens (SIL) structuresand in particular to techniques for constricting SIL structures, as wellas selected applications of such structures.

Due to the limitations on resolutions obtainable with conventionaloptical lenses for applications such as microscopy, techniques have beendeveloped to decrease the Rayleigh limit on transverse resolution δ. TheRayleigh limit is given by (δ=0.82λ/(NA) where λ is the wavelength andNA is the numerical aperture of the focusing, objective (NA=n sin (θ),where n is the refractive index of the medium, and θ is the anglebetween the outermost rays focusing on the sample and the optical axis)

Coherent light such as laser light can be used to precisely control thewavelength of illumination λ. One way to decrease the transverseresolution is to increase the index of refraction of the optical medium,Such as by use of oil-immersion microscopy or use of a solid immersionlens.

If an SIL is placed in contact with the sample under examination,illumination can be more readily focused on it, and use of the high NAof the system allows efficient collection of the excitation light withhigh optical transmission efficiency and observation of the sample witha very high resolution. In most of the cases, the SIL is used primarilyfor near-field microscopy, where the air gap between the SIL and thesample oblige those who do not want to use evanescent waves to work witha NA smaller than one.

A problem with the SIL technology is the complexity of its manufacture.For example, a polished glass sphere provided with a sequence ofprogressively finer alumina powders, requires a polishing time typicallyof many hours. Furthermore, the result is not perfect, and the polishedsurface is slightly rounded. Moreover, known lens structures in SILconfigurations involve objective lens sets that are self contained andthus are difficult to use in a manner that maintains the lens inimmersion contact with the object under observation.

What is needed is a method for simple, inexpensive and rapidconstruction of a solid immersion lens and a lens structure which issuited for low-cost, even disposable usage.

SUMMARY OF THE INVENTION

According to the invention, a solid immersion lens structure is formedof an optically clear low temperature moldable material such anelastomer cast to a desired shape and smoothness in a pliant mold whichhas highly undercut margins. Further according to the invention, amethod for construction of a solid immersion lens structure includesproviding a pliant mold defining a lens-shaped cavity in which a solidimmersion lens is cast, casting a liquid material into the lens cavity,permitting the liquid material to set to form the solid immersion lensportion and removing the solid immersion lens portion from the pliantmold with the highly undercut margins. A specific material for use asthe solid immersion lens is a thermally-resilient deformable materialsuch as optically-clear silicone elastomer of a refractive index ngreater than 1.2 and preferably greater than 1.4, such as a roomtemperature vulcanization (RTV) elastomer, specifically General ElectricRTV 615. Preferably, the mold itself may be constructed of this materialand the SIL structure can be a rigid setting material. The SILstructures according to the invention may be a disposable lens elementand/or a light collection element integrated with a specimen calTier ofa microfabricated flow cytometer.

According a specific embodiment of a method according to the invention,a first liquid elastomer such as RTV is injected into a container andallowed to solidify to a pliant elastomeric solid, then a smallnonreactive bead of the shape of the desired lens (a sphere) is placedon the first layer then partially covered with a layer of the liquidelastomer of a controlled thickness less than the diameter of the beadand allowed to solidify to stable pliancy. Thereafter the shaping beadis removed to yield a pliant smooth-walled mold of maximum diameter dwith highly undercut margins around an orifice. The mold and adjacentregion are then treated with an oxygen plasma to create a nonreactive,nonbinding surface interface. Then a third layer of optically-clearliquid moldable material, such as an RTV elastomer, having a thicknessslightly greater than the depth of the mold is injected into the moldand over the region around the orifice and then allowed to solidify. Theresultant structure is peeled from the pliant second layer to yield alens element in the form of a bead embedded on an attached flange,namely a solid immersion lens structure in accordance with theinvention. The pliant second layer is reusable as a mold.

Further according to the invention, a method is provided for imaging anobject using a low cost lens element in an SIL configuration. Accordingto this method, an object to be observed, preferably immersed in fluid,is guided along a passage defined by an integrally molded-together bodyportion and a solid immersion lens portion, where the solid immersionlens portion is optically aligned with a position in the passage. Theobject is positioned in the passage in alignment with the solidimmersion lens portion so that the object is within a field of viewextending through the spherical solid immersion lens portion. Theobject, immersed in a fluid of high index of refraction, is viewedthrough the spherical solid immersion lens portion of an even higherindex of refraction, and the object is imaged onto a viewing surface.

Further according to the invention, a method is provided for collectinglight emissions with high efficiency through a low cost lens element inan SIL configuration. An object to be observed is immersed in fluid andpositioned in alignment with the solid immersion lens portion so thatthe object is within a field of light collection extending through verylarge numerical aperture spherical solid immersion lens portion. Theobject, immersed in a fluid of high index of refraction, emitsobservable optical energy typically by fluorescence in response toexcitation, and the emissions at selected wavelengths are collectedthrough the spherical solid immersion lens portion of an even higherindex of refraction and directed to a sensor, typically without imaging,so that the emissions can be measured. The structure admits to highcollection efficiency.

In some embodiments, the solid immersion lens structure defines aninlet, leading into the passage, and an outlet, leading from thepassage. The object is guided along the passage comprising passing theobject through the inlet and along the passage. The object is supportedin a liquid. The liquid is pumped along the passage, thereby passing theobject through the inlet and along the passage.

Further according to the invention, a solid immersion lens structurecomprises a solid immersion lens portion with highly undercut marginsinterfacing on a flange portion, together with and a body portion inwhich there is a cavity or passage for carrying an object or sample tobe imaged or from which light is to be collected, where at least thesolid immersion lens portion is of a molded material formed in a mold ofpliant material with highly undercut margins.

The invention will be better understood by reference to the followingdetailed description and the accompanying diagrammatic drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-sectional view of a solid immersion lensstructure according to the invention.

FIG. 2 shows a schematic three-dimensional view of an initial step in amethod for producing a solid immersion lens structure in accordance withthe invention.

FIG. 3 shows a cross-sectional view corresponding to FIG. 2 indicatingfurther steps of producing a solid immersion lens structure inaccordance with the invention.

FIG. 4 shows a cross-sectional view corresponding to FIG. 3 showing asubsequent step in a method of producing a solid immersion lensstructure in accordance with the invention.

FIG. 5 shows an imaging system comprising a solid immersion lensstructure in accordance with the invention.

FIG. 6 shows an emission collection system comprising a solid immersionlens structure in accordance with the invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In order to understand the invention, it is helpful to define the termsassociated with a solid immersion lens SIL structure 50 as it might beused in a device such as a microscope, spectroscope or cytometer. FIG. 1illustrates the functioning of a solid immersion lens, with indicationof the parameters used to describe the structure and operation. A solidimmersion lens portion 51 comprises a sphere of radius r and index ofrefraction n_(s). It is disposed at a highest height h above a surface27 of a body portion 28 so that a boundary margin 25 is formed which isnarrower in diameter than the diameter of the lens portion 51. Anobservation region 52 is provided at a distance h′ from the surface 27.Samples are placed in the region for observation according to theintended application, such as microscopy, spectroscopy or cytometry.Also shown with the structure 50 is a collection/collimating lens 150.The spherical structure and collection configuration admits toconstruction of lens systems having a numerical aperture higher thanunity, which is particularly useful for ultrasensitive spectroscopy.

A method for producing a solid immersion lens structure in accordancewith the invention is described with reference to FIGS. 2-4. Referringto FIG. 2, a container 10, typically in the form of a shallow dishdefining a base wall 12 and four peripheral upstanding sidewalls 14provides the housing for a mold. To produce the solid immersion lensstructure in accordance with the method of the invention, a mold isformed. To form the mold, a first layer 16 of moldable material fromwhich the mold is to be formed is cast into the container 10. The firstlayer 16 is then permitted to set. Referring now to FIG. 3, once thefirst layer 16 has set, a mold core 18 is positioned in the container 10on the first layer 16. In this instance the mold core is a sphericalbead of uniformly smooth surface, such as a steel bead of radius r=˜0.8mm-4.5 mm. A second layer 20 of moldable material is then cast into thecontainer of height h=˜1 mm-5 mm partially to encapsulate the mold core18, thereby to form a second layer of moldable material 20 immediatelyadjacent the first layer 16. The second layer 20 defines an uppersurface 22 at height h such that an tipper portion 18.1 of the generallyspherical mold core 18 protrudes through an orifice 23 of diameter fromthe upper surface 22 and creates highly undercut margins 25 around theorifice 23. In order for the mold to be reusable where the structure hassuch undercut margins, the second layer, according to the invention,must be of a pliant material such as a silicone elastomer, such as aroom temperature vulcanization (RTV) elastomer, specifically GeneralElectric RTV 615.

The constraint on the height h is given by the following relation:

r(1−cosΦ)<h<r+r/n_(s)

where

r is the radius of the sphere,

h is the height of the layer,

Φ is the polar angle from the center of the sphere to the edge of theorifice formed by the undercut margins,

n_(s) is the index of refraction of the material which forms the lens.

Thus the geometric details of the mold depend upon the thickness of thesecond layer 20 relative to the radius of the bead. The RTV an elastomermade by mixing polymers, cross linkers and a catalyst. While it cures atroom temperature, it is typically set for two hours at a slightlyelevated temperature of 80° C. The preferred RTV comprises a first partof a polydimethylsiloxane bearing vinyl groups and a platinum catalystand a second part of a cross linker containing silicon hydride (Si—H)groups. Once mixed, the silicon hydride groups form a covalent bond withthe vinyl groups.

Referring to FIG. 4, once the layer 20 has set, the mold core 18 isremoved so as to define a lens cavity 24. In this manner, a mold 26 forproducing a solid immersion lens structure, in accordance with theinvention, is produced. To minimize the chance of bonding between themold and the lens, the surface is treated with an oxygen plasma to forman anti-adhesive layer 27. For example, oxidized RTV blocks bonding sothe molded lens can be removed from the lens cavity 24.

Still referring to FIG. 4 and to FIG. 1, the solid immersion lensstructure 50 is produced by casting a moldable material into the lenscavity 24. The moldable material from which the solid immersion lensportion of the solid immersion lens structure is to be formed istypically cast into the container 10 to fill not only the lens cavity24, but also to form a layer 28 in the container 10, the layer 28defining an upper surface 28.1 above (as shown) the lens cavity 24. Thethickness h′ above the surface 27 is given by the relation:

h′=r+r/n_(s)−h.

The layer 28 forms a body portion of the solid immersion lens structure50 when the moldable material of layer 28 has set. In this manner, thebody portion of the solid immersion lens structure is integrally moldedtogether with the solid immersion lens portion 51.

When the layer 28 has set, the solid immersion lens structure inaccordance with the invention, which includes a body portion 30 and asolid immersion lens portion 32 is formed. The solid immersion lensstructure is then removed from the mold.

The material from which the SIL structure 50 is made in mold 26 may beof any suitable optically clear material that can be cast as a liquid ata temperature less than the temperature at which the mold 26 is damagedor otherwise undesirably deformed. The SIL structure may cure to agenerally rigid solid or a pliant solid. Among the materials consideredto be generally suitable are low temperature of formation polymers, roomtemperature vulcanization elastomers, low temperature of formationepoxies, polyimides, polycarbonates and photoresists. The lens material50 can be a pliant silicone elastomer. A suitable silicone elastomer isGeneral Electric RTV 615, the same material used to create the mold 26itself.

As is clear from FIG. 4, the lens cavity 24 has a transverse dimensiongreater than the traverse dimension of an orifice 23 of the cavity 24.The moldable material from which the mold 26 is made is deformable sothat when the moldable material from which the solid immersion lensstructure 50 in accordance with the invention is made, has set, and isremoved from the mold 26, a region of the mold 26 adjacent the mouth ispermitted to deform thereby to permit the solid immersion lens portionto pass therethrough.

Referring now to FIG. 1, a solid immersion lens structure, generallyindicated by reference numeral 50, is indicated. The structure 50 hasbeen formed in accordance with the method of the invention as describedabove with reference to FIGS. 2-4 of the drawings. In addition, thestructure 50 has been formed to define a sample observation region 52 ina passage extending therethrough. This passage may be formed in anysuitable manner, such as by positioning an elongate mold core in thelayer 28, with reference to FIG. 4, prior to the moldable materialdefining layer 28 having set. The elongate mold core from which thepassage 52 is formed can be of a material which disintegrates whenexposed to a suitable agent. Accordingly, when the structure 50 has beenformed, the core can be removed by exposing it to the suitable agentthereby to remove the core from the structure 50 and to yield the hollowregion 52. Alternatively, the region 52 can be formed in any one of theways described in Applicants' co-pending U.S. patent application No.Ser. No. 09/605,520 filed Jun. 27, 2000. For example the region 52 canbe formed as part of a two-step construction process whereby the heighth′ is precisely defined during the first step and then the passage and abody portion is added as a second step.

The solid immersion lens structure defines an inlet 53 leading into thepassage 52 and an outlet 54 leading from the passage 52. A liquidsupports an object 55 in the passage 52. The liquid is pumped throughthe inlet 53 and along the passage 52 causing the object 55 to passthrough the inlet and along the passage 52 in the direction of thez-axis.

FIG. 5 is illustrative of an imaging system 100 employing an SIL 50 inaccordance with the invention. An example is a microscope. The systemincludes a laser 110 projecting a beam 111, an expansion lens 112, afirst collimating lens 114, a partially transmissive mirror 116, asecond collimating lens 118, an SIL structure 50, a focusing lens 120,an image detector such as a CCD camera 124 and a control apparatus 123.In operation, the laser 110 projects an illumination beam 111 throughexpansion lens 112 and collimating lens 114 to produce a broad coherentmonochromatic illumination beam 115. The beam 115 is reflected by mirror116 to second collimating lens 118 through which it is focused throughan air medium to the SIL structure 50. Focus adjustment is by means ofpositioning of the second collimating lens 118 relative to the SILstructure 50. The SIL structure 50 further focuses to a spot in thesample chamber (not shown) within the body portion, in immersion contactwith the lens. The sample is positioned by the control apparatus 123.(The control apparatus 123 may both position a platform and supply theobject or sample to be viewed.) Light reflected from the object isdirected back through the second collimating/focusing lens 118 whichfocuses to infinity and directs the image 119 through the half silveredmirror 116 to a third focusing/collimating lens 120. The thirdfocusing/collimating lens 120 focuses the image as magnified onto animage sensor 124, such as a CCD array of a CCD camera. The relativepositioning of the lens 120 and the image sensor 124 determines focus ofthe image. Other microscope configurations may be employed as suggestedby this configuration. Significantly, the SIL structure 50, although anessential element of the optical system is obtained from a manufacturingprocess which yields extremely inexpensive optical elements as comparedto conventional lenses, so the SIL structure 50, which is integral withthe sample carrier, is disposable. This is believed to be a significantadvance over conventional SIL technology. This also presents significantpractical advantages over methods using oil immersion objectives. It isnot necessary to use oil between the lens and sample since they areintegrally molded. In addition, an oil immersion objective must bepositioned with high accuracy with respect to the sample. However, asolid immersion lens can be fabricated as the appropriate distance awayfrom the sample so that focal precision would be needed to adjust thedistance between an oil immersion lens and the sample.

FIG. 6 is illustrative of a light collection system 200 employing an SIL50 in accordance with the invention. An example is a cytometer or ahighly efficient spectrometer. The system 200 includes a laser 210projecting a beam 211, an expansion lens 212, a first collimating lens214, an optional first dichroic filter 217 selected for passing theselected output wavelength of the laser 210, a partially transmissivemirror 216, a second collimating lens 218, an SIL structure 50, a seconddichroic filter 219 selected for passing the selected emission of thesample, a collection lens 220, a photon collection device such as aphotomultiplier tube 226 and a control apparatus 223. In operation, thelaser 210 projects an illumination beam 211 through expansion lens 212and collimating lens 214 to produce a broad coherent monochromaticillumination beam 215. Its purity is further selected by filter 217 sothat the illumination can be used as an excitation probe. The beam 215is reflected by mirror 216 to second collimating lens 218 through whichit is focused through an air medium to the SIL structure 50. Focusadjustment is by means of positioning of the second collimating lens 218relative to the SIL structure 50. However, as an emission collectionapparatus, imaging is not the goal. The SIL structure 50 furtherconcentrates the illumination to in the sample chamber (not shown)within the body portion, in immersion contact with the lens. The sampleis excited by the illumination and positioned by the control apparatus223. (The control apparatus 223 may both position a platform and supplythe object or sample to be viewed.) The illumination excites the sampleto cause it to emit fluorescent energy which is collected by the highnumerical aperture lens and is directed back through the secondcollimating lens 218 which focuses to infinity and directs the emittedphotonic energy through the half silvered mirror 216 to the secondfilter 219, which blocks any stray excitation, and then through thethird collimating lens 220. The third collimating lens 220 concentratesthe photonic energy into a collection region of a photon sensor such asa photomultiplier tube (PMT) 226. The relative positioning of the lens120 and the PMT 226 the collection efficiency. This application isbelieved to be a new application of an SIL structure. Otherconfigurations may be employed as suggested by this configuration.Significantly, the SIL structure 50, although an essential element ofthe optical system is obtained from a manufacturing process which yieldsextremely inexpensive optical elements as compared to conventionallenses, so the SIL structure 50, which is integral with the samplecarrier, is disposable. This is believed to be a significant advanceover conventional SIL technology.

The invention has been explained with reference to specific embodiments.Other embodiments will be evident to those of ordinary skill in therelevant art. It is therefore not intended that this invention belimited, except as indicated by the appended claims.

What is claimed is:
 1. A method for constructing a solid immersion lensstructure, the method comprising: producing a mold so as to define apliant lens cavity in which a spherical solid immersion lens portion ofthe solid immersion lens structure is to be formed, the pliant lenscavity having an orifice having a transverse dimension less than atransverse dimension of the lens cavity for the spherical solidimmersion lens portion; casting a moldable material, from which at leastthe solid immersion lens portion is to be formed, into at least the lenscavity; permitting the moldable material to set thereby to form thesolid immersion lens portion of the solid immersion lens structure; andremoving the solid immersion lens portion of the solid immersion lensstructure from the pliant mold after the moldable material has set.
 2. Amethod according to claim 1, wherein producing the mold comprisesforming the mold from a thermally-resilient deformable material.
 3. Amethod for constructing a solid immersion lens structure, the methodcomprising: producing a mold so as to define a pliant lens cavity inwhich a spherical solid immersion lens portion of the solid immersionlens structure is to be formed, the pliant lens cavity having an orificehaving a transverse dimension less than a transverse dimension of thelens cavity for the spherical solid immersion lens portion; wherein themold producing step comprises casting a first catalyzed crosslinkableliquid around a mold core in the form of a generally spherical elementuntil it is set as the mold, thereafter removing the mold core from themold after the first catalyzed crosslinkable liquid has set wherein aregion of the mold adjacent the orifice is allowed to temporarily deformthereby to permit the spherical element to be removed with minimaldamage to the mold; thereafter casting a second catalyzed crosslinkableliquid into the mold until it has set; and then removing the solidimmersion lens portion from the mold after the second catalyzedcrosslinkable liquid has set wherein the region of the mold adjacent theorifice is allowed to temporarily deform thereby to permit the solidimmersion lens portion to be removed.
 4. A method for constructing asolid immersion lens structure, the method comprising: producing a moldso as to define a pliant lens cavity in which a spherical solidimmersion lens portion of the solid immersion lens structure is to beformed, the pliant lens cavity having an orifice having a transversedimension less than a transverse dimension of the lens cavity for thespherical solid immersion lens portion; wherein the mold producing stepcomprises casting a first moldable material into a container to form afirst layer from which the mold is to be formed and permitting the firstlayer to set, then positioning the mold core on the first layer, andthen casting the first moldable material into the container to at leastpartially encapsulate the mold core, thereby to form a second layerimmediately adjacent the first layer.
 5. A method according to claim 4wherein the generally spherical mold core protrudes from the uppersurface of the first layer.
 6. A method according to claim 5 furthercomprising casting a second moldable material from which at least thesolid immersion lens portion of the solid immersion lens structure is tobe formed into the container to fill not only the lens cavity, but alsoto form a layer in the container, the layer defining an upper surfaceabove the lens cavity, such that the layer forms a body portion of thesolid immersion lens structure when the second moldable material hasset, the body portion then being integrally molded together with thesolid immersion lens portion.
 7. A method according to claim 6 furthercomprising coating the lens cavity and the upper surface of the moldwith a release agent to inhibit adhesion of the solid immersion lensportion and the body portion of the solid immersion lens structure tothe mold at the upper surface and in the lens cavity.
 8. A methodaccording to claim 6 further comprising forming a channel adjacent thelayer defining the upper surface above the lens cavity before the firstmoldable material has set.
 9. A method according to claim 4 wherein thefirst moldable material from which at least the first layer is formedcomprises a silicone elastomer.
 10. A method according to claim 9wherein the second moldable material from which at least the lensportion is formed comprises a silicone elastomer.
 11. A method forimaging an object, the method comprising: guiding the object along apassage defined by an integrally molded together solid immersion lensstructure, the solid immersion lens structure defining a body portion,in which the passage is defined, and a solid immersion lens portion, thesolid immersion lens portion being optically aligned with a position inthe passage; positioning the object in the passage at the position whichis optically aligned with the solid immersion lens portion so that theobject is within a field of view extending through the solid immersionlens portion; and viewing the object in the passage through the solidimmersion lens portion while the object is at the position in thepassage which is optically aligned with the solid immersion lensportion.
 12. A method as claimed in claim 11, wherein the solidimmersion lens structure defines an inlet, leading into the passage, andan outlet, leading from the passage, guiding the object along thepassage comprising passing the object through the inlet and along thepassage.
 13. A method as claimed in claim 12, wherein the object issupported in a liquid, passing the object through the inlet and alongthe passage comprising pumping the liquid along the passage.
 14. Amethod as claimed in any one of claims 13, wherein viewing the objectthrough the solid immersion lens while at the position in the passagewhich is optically aligned with the solid immersion lens portioncomprises optically aligning a microscope with the solid immersion lensportion, and viewing the object through the microscope.